staging: comedi: das08: rename DAS08_OP() and DAS08_DO_MASK

The `DAS08_DO_MASK` macro is a bitmask for the control register
corresponding to the digital output channels (except on "JR" boards).
Rename it to `DAS08_CONTROL_DO_MASK` and add a comment.

The `DAS08_OP(x)` macro takes a bitvector of the desired digital output
channel states and returns the corresponding bits for the control
register (except on "JR" boards).  Rename it to `DAS08_CONTROL_DO(x)`
and add a comment.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>